Using non-carbon anodes—i.e. anodes which are not made of carbon as such, e.g. graphite, coke, etc . . . , but possibly contain carbon in a compound—for the electrowinning of aluminium should drastically improve the aluminium production process by reducing pollution and the cost of aluminium production. Many attempts have been made to use oxide anodes, cermet anodes and metal-based anodes for aluminium production, however they were never adopted by the aluminium industry.
For the dissolution of the raw material, usually alumina, a highly aggressive fluoride-based electrolyte, such as cryolite, is required.
Materials for protecting aluminium electrowinning components have been disclosed in U.S. Pat. Nos. 5,310,476, 5,340,448, 5,364,513, 5,527,442, 5,651,874, 6,001,236, 6,287,447 and in PCT publication WO01/42531 (all assigned to MOLTECH). Such materials are predominantly made (more that 50%) of non-oxide ceramic materials, e.g. borides, carbides or nitrides, and are suitable for exposure to molten aluminium and to a molten fluoride-based electrolyte. However, these non-oxide ceramic-based materials do not resist exposure to anodically produced nascent oxygen.
The materials having the greatest resistance to oxidation are metal oxides which are all to some extent soluble in cryolite. Oxides are also poorly electrically conductive, therefore, to avoid substantial ohmic losses and high cell voltages, the use of oxides should be minimal in the manufacture of anodes. Whenever possible, a good conductive material should be utilised for the anode core, whereas the surface of the anode is preferably made of an oxide having a high electrocatalytic activity.
Several patents disclose the use of an electrically conductive metal anode core with an oxide-based active outer part, in particular U.S. Pat. Nos. 4,956,069, 4,960,494, 5,069,771 (all Nguyen/Lazouni/Doan), 6,077,415 (Duruz/de Nora), 6,103,090 (de Nora), 6,113,758 (de Nora/Duruz) and 6,248,227 (de Nora/Duruz), as well as PCT publications WO00/06803 (Duruz/de Nora/Crottaz), WO00/06804 (Crottaz/Duruz), WO00/40783 (de Nora/Duruz), WO01/42534 (de Nora/Duruz) and WO01/42536 (Nguyen/Duruz/ de Nora).
U.S. Pat. Nos. 4,039,401 and 4,173,518 (both Yamada/Hashimoto/Horinouchi) disclose multiple oxides for use as electrochemically active anode material for aluminium electrowinning. The multiple oxides include inter-alia oxides of iron, nickel, titanium and yttrium, such as NiFe2O4 or TiFe2O4, in U.S. Pat. No. 4,039,401, and oxides of yttrium, iron, titanium and tantalum, such as Fe2O3.Ta2O5, in U.S. Pat. No. 4,173,518. The multiple oxides are produced by sintering their constitutive single oxides and then they are crushed and applied onto a metal substrate (titanium, nickel or copper) by spraying or dipping. Alternatively, the multiple oxides can be produced by electroplating onto the metal substrate the constitutive metals of the multiple oxides followed by an oxidation treatment.
Likewise U.S. Pat. Nos. 4,374,050 and 4,374,761 (both Ray) disclose non-stoichiometric multiple oxides for use as electrochemically active anode material for aluminium electrowinning. The multiple oxides include inter-alia oxides of nickel, titanium, tantalum, yttrium and iron, in particular nickel-iron oxides. The multiple oxides are produced by sintering their constitutive single oxides and then they can be cladded onto a metal substrate.
WO99/36591 (de Nora), WO99/36593 and WO99/36594 (both Duruz/de Nora) disclose sintered multiple oxide coatings applied onto a metal substrate from a slurry containing particulate of the multiple oxides in a colloidal and/or inorganic polymeric binder, in particular colloidal or polymeric alumina, ceria, lithia, magnesia, silica, thoria, yttria, zirconia, tin oxide or zinc oxide. The multiple oxides include ferrites of cobalt, copper, chromium, manganese, nickel and zinc. It is mentioned that the coating can be obtained by reacting precursors thereof among themselves or with constituents of the substrate.
U.S. Pat. No. 6,372,119 and WO01/31091 (both Ray/Liu/Weirauch) disclose a cermet made from sintered particles of nickel, iron and cobalt oxides and of metallic copper and silver possibly alloyed with cobalt, nickel, iron, aluminium, tin, niobium, tantalum, chromium molybdenum or tungsten. The particles can be applied as a coating onto an anode substrate and sintered thereon to form an anode for the electrowinning of aluminium.
These non-carbon anodes have not as yet been commercially and industrially applied and there is still a need for metal-based anodes for aluminium production.